Public Policy Polling (PPP), a Democratic-leaning firm, looked at the approval ratings for 59 senators during the second half of 2010, and several of them will face voters in 2012. The senators were ranked by the spread between their approval numbers and disapproval numbers.

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Of the 59, Klobuchar is the most popular, with an approval rating of 59 and a disapproval of 29 (a 30-point spread), while Lieberman has an approval rating of 33 and a disapproval of 54 (a 21-point spread).

Coming in second was Sen. Scott Brown (R-Mass.), who's also up in 2012. He had a 53 percent approval and 29 percent disapproval (a 24-point spread). Sen. Judd Gregg (R-N.H.), who is retiring, was third, and Sen. Olympia Snowe (R-Maine), also up in 2012, was fourth, with a 56 percent approval and 34 percent disapproval (a 22-point spread).

Retiring Sen. Roland Burris (D-Ill.) came in last, with a 19 percent approval rating and a 58 percent disapproval rating.

PPP's Tom Jensen notes in his analysis that "the average approval rating for a senator right now is 43 percent approving to 40 percent disapproving" and that "there's very little difference in the average approval of Republican senators versus Democratic senators." The average GOP senator had a 43/37 split, while the average Democrat rated a 44/41.